Regulations on an exhaust gas are intensified year by year in respective countries and with recent and growing concern over the environmental conservation, such regulations are expected to become more strict in the future. Conventionally, in order to provide a high-performance catalyst having an excellent exhaust gas cleaning performance, not only improvement of the catalyst performance by changing the combination or the like of catalyst active components, that is, noble metals, but also high loading of a noble metal on a catalyst carrier are generally performed.
However, high-performance formulation of a catalyst incurs an increase in the amount of a noble metal used or a growth in the catalyst size, which leads to an increase in the catalyst weight and further causes a problem of cost rise.
In addition, the price of a noble metal tends to elevate every year. Accordingly, development of a carrier or catalyst capable of enhancing the exhaust gas cleaning performance without increasing the amount of a noble metal loaded is being aggressively made in recent years. For example, Kokai (Japanese Unexamined Patent Publication) No. 5-195763 discloses a tandem•metal catalyst carrier where two metal honeycomb bodies are tandemly disposed with spacing in an outer casing and the coupling site of the metal honeycomb carrier on the downstream side to the outer casing works out to the inlet end of the metal honeycomb carrier on the downstream side, thereby enhancing the exhaust gas cleaning performance without increasing the amount of a noble metal loaded.